Dump trucks, etc. use a planetary gear speed reducer in which a large speed reduction ratio can be obtained. Planetary rotary members provided in such a planetary gear speed reducer comprise planetary gears or planetary rollers rotating about their respective axes while revolving around a gear, and supported by pins of a carrier through rolling bearings. Tapered roller bearings are used as such rolling bearings.
The centrifugal force generated from the center of the bearing when the planetary rotary member rotates about its axis, and the centrifugal force generated from the center of the speed reducer, etc. when the planetary roller revolves around a gear are applied to the retainer of such a tapered roller bearing supporting the planetary rotary member. The latter centrifugal force due to the revolution of the planetary rotary member is applied to the retainer such that the retainer is eccentric from the center of the bearing. When the speed of the planetary rotary member revolving around a gear becomes faster so that a centrifugal force becomes larger, if the retainer is guided by rolling elements, the eccentric retainer strongly brings some pillars of the retainer into abutment with tapered rollers. As a result thereof, the abutting pillars may become worn or break due to the bending moment concentrating on the roots of the pillars coupled to the annular portions of the retainer. In order to avoid this, a retainer guided by a raceway is disclosed (in the below-identified Patent documents 1 and 2).
Patent document 1 discloses a tapered roller bearing used in a planetary gear speed reducer, and including an outer race formed on its outer diameter surface with a retainer guiding surface, and a retainer formed with a small-diameter annular portion and a large-diameter annular portion. A guided surface is formed on at least one of the small-diameter annular portion and the large-diameter annular portion so as to come into sliding contact with the retainer guiding surface.
Patent document 2 discloses a tapered roller bearing including a retainer made of a steel plate and formed with a small-diameter annular portion. Guided surfaces are formed at the distal ends of protrusions formed by cutting and raising some sections of the small-diameter annular portion.
The retainer of Patent document 2 is guided at only the protrusions of the small-diameter annular portion when guided by the inner race or the outer race. Therefore, when the tapered roller bearing is used in the situation in which a large centrifugal force is generated, e.g., used to support a planetary rotary member, the retainer tends to tilt so that the contact area of the guided surfaces of the retainer decreases or the pillars interfere with the tapered rollers.
On the other hand, the contact area of the guided surface of the retainer disclosed in Patent document 1 is larger than that of the guided surfaces of the retainer disclosed in Patent document 2. Therefore, it is possible to prevent wear and seizure of the guided surface by guiding the retainer with a low contact surface pressure. If a retainer is guided by an outer race as in Patent document 1, compared to the arrangement in which the retainer is guided by an inner race, the circumferential length of the guided surface of the retainer is long so that the contact area of the guided surface is large.